1. Field of the Invention
This invention relates broadly to magnetically shielded electronic cameras of the image dissector and similar types. More particularly, this invention relates to magnetically shielded electronic cameras adapted to be mounted moveably through both axial and radial magnetic fields, including earth's magnetic field and strong electrical field sources, yet operate accurately and reliably in high-gain electro-optical gaging systems.
2. Description of the Prior Art
Computerized steel bar rolling mills constantly require size and profile measurements be made of hot bar moving at speeds up to 4000ft./min. (1219 m./min.). Such bar measurements are frequently made by electro-optical gaging systems having one or more electronic cameras mounted moveably to scan the periphery of the hot moving bar. In commercially available systems, the electronic camera ofttimes includes an image dissector tube operating in a high-gain electronic circuit that will provide suitable sensitivity to detect bar size and profile changes for computer control purposes. However, these electronic cameras have not proved entirely satisfactory in certain steel mill installations for several reasons.
For example, in steel bar rolling mills for producing bar sizes in the range of 3 inches (7.62 cm.), large horsepower mill drive motors are used which are fed by SCR-fired motor controllers. The electronic camera(s) for the bar gaging system must usually be placed near one or both of these devices in order to accurately measure bar size and/or profile. This has the disadvantage of subjecting the very sensitive electronic camera to the effects of a strong erratic electrical magnetic field generated by the mill drive motor and/or its SCR-fired controller. These effects are further compounded during electronic camera movement through these magnetic fields as well as the earth's magnetic field during peripheral scanning of the moving bar to develop profile data. The combined effect on present day commercially available image dissector type electronic cameras is to erratically vary the focal plane as well as the image size of the photocathode as it is being projected upon the electron aperture plane. Thus, for a standard size bar being detected by the electronic camera, the camera output signal may vary to signify a change in bar size when in fact the standard bar size had not changed at all. For instance, it was observed and verified by theory that a change in axial field of one gauss at the photocathode caused a one percent change in the image size. This produces an unpredictable error in bar measurements which is worse when the camera is moving than when held stationary.